Topic 5: energy changes

What happens when a chemical reaction occurs

• energy is conserved (same at the beginning as it is at the end)

What does it mean if a reaction transfers energy to the surrounding

The product molecules have less energy than the reactants

Exothermic reactions

• Transfers energy to the surroundings increases

• Product molecules have less energy than reactants by amount transferred

Examples of Exothermic reactions

• combustion

• Most oxidation reactions

• Neutralisation

• Salt heating cans

• Hand warmers

Endothermic reactions

• takes in energy from surrounding so temperature of surrounding decreases

• Product molecules have more energy than reactants

Examples of endothermic reactions

• thermal decomposition

• Reaction of citric acid and sodium hydrogen carbonate

• Sports injury packs

Reaction profiles

• used to show relative energies of reactants and products

Activation energy

Minimum amount of energy needed for particles to react

Energy change of reactions

• energy must be supplied to break bonds in the reactant

• Energy is released when bonds in the product are formed

Calculating overall energy change

Sum of energy taken in to break bonds - sum of energy released to form bonds

Cells

• contain chemicals which react to produce electricity

• Voltage produced by cell is dependant upon factors eg type of electrode

• Simple cell can be made by connecting two different metals in contact with electrolyte

Batteries

Two or more cells connected together in series to provide a greater voltage

Non rechargeable cells and batteries

• chemical reaction stops when one reactant has been used up (limiting reactant)

• Alkaline batteries are non rechargeable

Rechargeable cells and batteries

Can be recharged as chemical reactions are reversed when an external electrical current is supplied

Fuel cells

• Supplied by external source of fuel (such as hydrogen) and oxygen or air

• Fuel is oxidised electrochemically within the fuel cell to produce potential difference

• Overall reaction of hydrogen fuel cells involves oxidation of hydrogen to produce water

• Offers potential alternatives to rechargeable cells and batteries

Hydrogen fuel cells factors

• fuel cells can be constantly used provided that fuel keeps being added

• Only produces water when burnt

• Needs to be stored at a high pressure so it is harder to transport

Rechargeable cells and batteries factors

• can be recharged by reversing reaction so fuel doesn’t need to be supplied constantly

• Hard to dispose of (non- biodegradable)

• Will eventually stop working